1. Field of the Invention
The present invention relates to the field of photomultipliers.
2. Prior Art
Growing demand on photo detectors capable of detecting small intensity light fluxes and counting of single photons facilitated designing novel solid state detectors. One of them is the so-called silicon photomultiplier (SiPM), which was claimed to be able to replace traditional photomultiplier tubes (PMT). Among the possible areas of application of such detectors are high energy physics and nuclear medicine, in which brief detection of a small number of optical photons produced by a single γ-quantum in a scintillator crystal is required. Due to the features of their structure, SiPM have advantages of low operating voltage (<50 V), fast response time (˜100 ps), small size, etc. However, currently available designs have drawbacks that limit their application (see below).
A silicon photomultiplier (SiPM) is a device, which is in fact a large number of small SPAD (single photon avalanche diode) pixels, connected in parallel to a single output circuit. The detailed description of the conventional device is presented in several recent works which describe the front illuminated device. Its operation is based on the idea that very small SPAD pixels, if assembled in a dense, two-dimensional (2D) array, can be fired separately by a single photon each. Therefore, the overall signal output is proportional to the number of pixels fired at a time and the dynamic range of such 2D array of m elements is proportional to m.
One of the possible versions of a conventional front-illuminated structure is presented in FIG. 1. The structure of each pixel can be either a regular reach-through structure built on a p-type substrate (as it is shown in FIG. 1), or the other possible version that does not require reach-through effect. For example, the structure built on n-type substrate with a buried p+ layer can be applied. The operating voltage is several volts above the avalanche threshold, which drives each pixel in a Geiger operation mode.
The structure in FIG. 1 is built using a p-type substrate 1 with a thin epi layer 2. The Geiger mode avalanche pixels are represented by the cathode diffusion region 3 and avalanche region 4. The substrate 1 serves as the anode. The junction depth 1 is small enough to maximize quantum efficiency in the short wavelength range. The photo-generated carriers can be collected either from the avalanche region 4 solely, or from both the avalanche region 4 and depleted intrinsic layer w.
Two pixels of SiPM shown in FIG. 1 can be fired simultaneously by two different photons. To avoid optical crosstalk generated by hot carriers in avalanche region, the narrow trenches 10 are made between the pixels to block the photons generated within the avalanche area of one pixel against reaching the neighboring pixels. Oxide layer 11 with Si interface forms a reflective and isolation layer. The trenches may be filled with light absorptive material. Each pixel is loaded with a resistive layer 12 that quenches the avalanche when the pixel photocurrent exceeds a certain level. The quenching time is important parameter since it determines the pixel recovery time. The cathode metal contacts 20 from each pixel of SiPM are connected together and are hooked to the SiPM front-end electronics (not shown in FIG. 1). The back surface of SiPM is coated with metal 21 that serves as the anode contact.
Since the active volume of each pixel can be made very small (thin epi-layer d in FIG. 1), the thermally generated noise current can be minimized to the level when it does not interfere with detection of optical photons.
Among the drawbacks of the conventional front-illuminated structures like shown in FIG. 1, are the low quantum efficiency for the short wavelengths (<500 nm) and relatively small fill factor (FF). The latter parameter value is limited by the requirement to reserve a part of active area space for the avalanche quenching resistive layer, contacts, and metal traces.
Back-illuminated versions of SiPM may provide advantageous solutions since back-illuminated detectors have inherently higher quantum efficiency at the short wavelengths of light and FF is not limited by the resistive layer and metal traces, which are deposited opposite to the light incident surface of the device. The example of the back-illuminated SiPM integrated with Si drift detector is described in reference 16 (see FIG. 2). However, structures like shown in FIG. 2 have a significant drawback: These structures are designed on thick, bulk Si wafers and a significant amount of thermally generated carriers hinder detection of optical photons.